1. Field of the Invention
This invention relates to electron beam lithography and particularly to utilizing an electron anti-fogging baffle as a detector.
2. Description of the Related Art
Electron beam lithography typically uses electrons accelerated in vacuum to expose a resist by penetrating the resist layer. When an electron beam of sufficient energy (e.g., 50 keV) impinges on a substrate, such as in a lithographic exposure of a resist-coated mask, electrons are emitted from the mask surface. Lower level energy electrons are typically referred to as “secondary electrons” while higher energy level electrons are referred to as “backscattered electrons.” Most of the secondary electrons have an energy level below 50 eV, whereas most of the backscattered electrons have an energy level above at least 20 keV.
Conical baffles placed near the substrate surface are often used to control the effects of these scattered electrons. One use of such baffles is to reduce charging-induced beam drift, as taught in U.S. Pat. No. 5,838,006, incorporated herein by reference in its entirety. The conical baffles are arranged such that scattered electrons impinge on baffle surfaces that are not directly seen by the primary beam. Because these baffle surfaces are not directly seen by the primary beam, charge accumulation and hydrocarbon film formation thereon do not significantly affect the primary beam.
Scattered electrons from the substrate surface can also strike the bottom of the objective lens and excite additional scattered electrons (“objective scatter”) that can expose the resist in a diffuse way. This additional exposure, generally referred to as fogging, can disturb the fidelity of the main exposure. This fogging can be controlled to some degree by an arrangement of multiple (concentric) conical baffles, as taught in U.S. Pat. No. 6,326,635, incorporated herein by reference in its entirety. The effect of such in arrangement is to provide controlled paths for the scattered electrons designed to reduce the probability of scattered electrons striking the objective lens (thereby causing objective scatter) and/or reduce the probability of objective scatter returning to the mask. To help accomplish this, the baffles may be arranged in a manner that presents a minimal cross-sectional area to the substrate-scattered electrons. The minimal cross-sectional area may help the baffles collect substrate-scattered electrons by guiding them into vanes where they are absorbed by baffle surfaces.
The amount of electrons scattered from a mask surface, and collected by the baffle, will depend on the particular features of the surface. As an example, different geometrical features and textures may result in varying amounts of scattered electrons. As a result, a useful image of the surface could be formed if the amount of collected electrons could be plotted as the beam is scanned over some or all of the mask surface. However, there is currently no available mechanism for easily capturing this information. As a result, to generate such an image in a conventional beam writing system, specialized sensors are required.
However, such specialized sensors add cost and complexity, which may not be justified for a system whose primary purpose is writing. Accordingly, what is needed is an improved anti-fogging baffle arrangement that allows the amount of scattered electrons collected therein to be captured.